The ability of phagocytes to produce large amounts of oxygen radicals constitutes an important host-defense mechanism against microbial infection. However, the same oxidant-generating machinery also contributes to tissue injury such as in Acute Lung Injury (ALI) resulting from uncontrolled activation of phagocytes. While research on phagocyte NADPH oxidase has produced a great deal of information regarding its activation mechanisms, little is known about the negative regulatory mechanisms that limit phagocyte oxidant production. The objective of Project 2 is to define such negative regulatory mechanisms and explore the potential for dampening lung inflammatory response. Studies are proposed in two substantial specific aims that focus on our recently identified regulatory mechanism by MAP kinase phosphatase 5 (MKPS), which restrains oxidant production by polymorphnuclear neutrophils, thereby decreasing LPS-induced inflammatory vascular injury. In Aim 1, we will determine how MKP5 regulates PMN oxidant production, which is critical to ALI. The profound negative regulatory effect by MKP5 suggests that p38 MAPK, the target of MKP5, is critical to PMN NADPH oxidase activation. Studies are proposed to test the hypothesis that p38 MAPK is essential for sequential phosphorylation of p47[phox] by a multitude of protein kinases, leading to p47[phox] conformational change and full activation of NADPH oxidase. We will also test the hypothesis that p38 MAPK signaling is further amplified through MK2, a downstream effector and protein kinase that contributes to p47[phox] phosphorylation. The in vivo function of MK2 in ALI and its regulation by MKP5 will be interrogated with the use of MK2 -/- and MKP5 -/- mice. In Aim 2, we will define the central role of MKP5 in preventing ALI through modulation of PMN and endothelial activation. Using mouse models of lung inflammation, we will test the hypothesis that MKP5 is an essential regulator that reduces proinfiammatory cytokine and chemokine expression, limits PMN infiltration, and dampens PMN oxidant production and oxidant-mediated lung injury. We will also query the possibility that endothelial MKP5 is important in limiting ICAM-1 expression and thus prevents PMN adhesion as well as oxidant-mediated injury. Together, these studies aims to delineate the underlying mechanism for LPS priming of PMN oxidant production, and explore a novel negative regulatory mechanism for its therapeutic potential in controlling ALI.